This invention has to do with stress wave generation in the earth and more particularly is concerned with method and apparatus for the controllable non-explosive generation of longitudinal and shear stress waves in an earth zone surrounding a bore hole.
The strength characteristics of the earth surface layer are of interest in determining suitability of various sites for construction and for like decisions premised upon seismic considerations. Additionally, the geological features of the earth are of interest from a minerals exploration point of view, and for these purposes it has been the practice to evaluate samples of the earth drawn from locations of interest for a wide variety of strength properties and similarly, to conduct seismic tests over relatively larger portions of the earth surface to detect subsurface location of e.g. oil sands.
The measurement of earth properties by obtaining and removing a sample from the earth to a laboratory for a variety of tests requires in decision making a number of assumptions concerning the relationships of the test data to the actual on-site properties, which of course are altered by removal of the sample from the earth. The conduct of on-site earth property evaluations has been hampered by the lack of equipment suited for on-site generation of the stresses necessary to measure properties of interest. Heretofore, stress generators have primarily been laboratory instruments. On-site generation of stress waves is practiced in a variety of instances, but has heretofore generally required the use of explosive devices such as packages of TNT, if stress waves of sufficient strength and intensity over large enough distances to insure typical medium response were to be obtained. Subsurface explosive tests are disfavored from an environmental point of view, further encumbering the earth testing process.
Additionally, explosively generated stress waves are essentially longitudinal in nature by virtue of the explosive characteristic, although frequently the property of interest in the earth is a property related to its performance in shear. For this purpose, shear stress waves are needed, and again resort must be had to the laboratory scale of equipment to obtain shear stress waves, and thus shear data, with respect to an earth sample. Again, the benefits of on-site testing are lost.